1. Field of the Invention
The present invention relates to superplastic aluminum alloys and particularly to an improved method for producing such materials.
2. Description of the Prior Art
Efforts to produce improved superplastic aluminum alloys, i.e., alloys of aluminum which can be superplastically formed using gas pressure or vacuum have been numerous and extensive as evidenced by the plethora of prior art describing such materials and methods for their preparation.
Among this prior art, two relatively recent techniques appear to produce the most desirable (i.e., commercially valuable) superplastic materials.
The first of these techniques is described in U.S. Pat. No. 3,847,681 issued Nov. 12, 1974, to Waldman et al. This technique, which is presented schematically in FIG. 1 hereof, involves the steps of:
(a) solution heat treating the starting material for from 8-48 hours at a temperature greater than 860.degree. F.; PA0 (b) slow cooling the product of step (a) to an overage temperature, i.e., about 775.degree. F.; PA0 (c) overaging at about 775.degree. F. for 3 to 5 hours; PA0 (d) slow cooling the product of step (c) to a temperature of between about 450.degree.-500.degree. F. and optionally holding at this temperature for up to 4 hours; PA0 (e) plastically deforming the material (from 40-80%) at a temperature between about 450.degree. and 500.degree. F.; and PA0 (f) rapidly recrystallizing at a temperature of between about 800.degree. and 900.degree. F.
This process reportedly provides a fine grain structured 7000 series alloy.
A photomicrograph of 7475 alloy prepared using this procedure is shown in FIG. II. As is clear from this picture, although the grains are relatively fine, their aspect ratio, i.e., length to width ratio, is quite high.
The second prior art process which produces acceptable material is that described in U.S. Pat. No. 4,092,181 issued May 30, 1978 to Paton et al. This patent describes a process for preparing material reportedly of finer grain than that described in the '681 patent, according to a somewhat shorter procedure, and in heat treatable alloys other than those of the '681 patent, which additional alloys may include chromium as an alloying element.
The process of the '181 patent is quite similar to that of the '681 patent except that it offers the option of cold water quenching after solution heat treat and before overage (i.e., between steps (a) and (c) of '681) and eliminates the need entirely for the optional soaking or holding of step (d) of the '681 patent.
In each of these references, the mechanical work required to induce the lattice strain necessary for recrystallization is performed while the material is warm, i.e., at between 400.degree. and 650.degree. F. Although the '181 patent alludes to the feasibility of performing such work at lower temperatures, i.e., "below the overage temperature" there is no disclosure of "cold" rolling, i.e., rolling at room temperature.
Both of the foregoing processes provide useful superplastic materials as evidenced by evaluation thereof by the inventors of the present process. These prior art processes are, however, somewhat difficult to work into a commercial production operation because of the apparent requirement that substantially all of the mechanical work be accomplished in a hot or warm condition.
The term "and/or" as used herein is meant literally, i.e., when referring to the use of steps A and/or B, is meant using A and B, or B alone, but not A alone.
The advantages of fine and equiaxed grain structure in superplastic materials are discussed in detail in "Superplasticity", J. W. Edington, K. N. Melton and C. P. Cutler, Progress in Materials Science, vol. 21, No. 2, pp. 63-170, Pergamon, N.Y. (1976).